1. Field of the Invention
The present invention relates generally to heat sinks for electronics assemblies, such as RF power amplifiers. The invention further relates to methods of manufacture of electronics assemblies incorporating heat sinks.
2. Description of Related Art
Numerous electronic processes generate waste heat. For example, in the case of RF power amplifiers for cellular phone base stations, the amplification methods used to achieve the required linearity and low noise operation produce large amounts of waste heat which must be removed in order to preserve the efficiency, operating characteristics and the life of an amplifier. In most applications, this waste heat must be dissipated via heat transfer to ambient air forced through the electronics assembly by a fan. Due to the low heat transfer rate to air via convection, however, the ability to transfer heat merely by fan cooling is inadequate for many applications. Heat sinks serve to greatly improve the waste heat transfer rate by providing large surface areas, such as fins, to dissipate the heat into the air. Therefore, large area heat sinks are critical for many applications where significant heat is generated by the electronics assembly.
Important characteristics of such heat sinks are low cost, light weight, ease of assembly and good thermal transfer properties. These characteristics are difficult to achieve together, however, since good thermal transfer properties typically require relatively expensive materials and manufacturing techniques. In particular, good thermal transfer properties typically require a heat sink material with high thermal conductivity to efficiently transfer heat to the fins, and thin closely spaced fins to maximize contact area with the air. These features, however, typically require a more expensive material and/or a more expensive manufacturing process. This may be best appreciated by considering specific examples of common heat sink structures.
The least expensive type of heat sink is the die cast heat sink. A typical die cast heat sink includes a base and heat dissipation fins as a single integral structure made of the same die cast material, such as aluminum. The die cast heat sink is a very desirable manufacturing method from a cost standpoint as the majority of mounting features, clearance holes, shielding features, etc., can be directly cast into the base eliminating costly secondary machining operations. The die casting process, however, inherently limits the thermal conductivity of the material as well as the thickness of the fins and the spacing between the fins. These limitations prevent die cast heat sinks from being used for high power density electronics assemblies, such as RF power amplifiers, which require greater thermal performance.
Extruded heat sinks are commonly used for high power amplifiers because the extrusion process provides better thermal conductivity due to reduced porosity in the metal and also enables a reduced fin thickness and spacing over die casting. Although extruded heat sinks achieve the desired fin thickness and spacing, extensive additional machining is required to create mounting features, clearance holes and shielding features, etc., in an extruded heat sink. Also, most extruded heat sinks include some grade of aluminum because of its relatively low cost, manufacturability and heat transfer characteristics. Aluminum does not have a sufficiently high conductive heat transfer rate, however, for more demanding applications such as some RF power amplifiers.
FIG. 1 is a side elevation view of such an extruded heat sink according to the prior art. The heat sink 5 comprises a base 6 that is integral with the fins 7, which is typically extruded and then machined. As noted above, it is formed out of an aluminum material having insufficient thermal conductivity for demanding applications, such as RF power amplifiers. Therefore, an additional plate 8 of a material having a higher thermal conductivity, such as copper, is disposed between the sink and the electronics assembly generating the heat. Without the additional intermediary plate 8, the heat sink 5 would not be able to absorb and dissipate the heat from the electronics assembly quickly enough. The copper plate 8, however, adds to the cost and weight of the heat sink. Furthermore, while the copper plate 8 serves to spread the heat quickly through the aluminum base 6, nonetheless the lower heat transfer rate of the aluminum reduces the heat transfer efficiency out into the fins 7. Therefore, despite the relatively high cost of the heat sink of FIG. 1, its performance is still not as good as desired for some applications.
Therefore, what is needed is a heat sink that can provide the above noted characteristics of low cost, light weight, ease of assembly and good thermal transfer properties, which characteristics have been mutually incompatible in the prior art.